For evolution to occur, developmental processes must be modified. However, the relationship between development and evolution is only beginning to receive experimental study. We have defined an experimental system that allows us to study the evolution of pathways of early development. The proposed research is to investigate the changes in cellular behavior and gene expression that accompany a radical evolutionary change from the ancestral indirect mode of development of typical sea urchins to direct development. The experimental system is a molecular and developmental comparison of a direct developing sea urchin, Heliocidaris erythrogramma, to two closely related typical developing species, H. tuberculata and Strongylocentrotus purpuratus. The major problem to be addressed is how the two developmental pathways diverge at the end of gastrulation. The gastrula of H. erythrogramma resembles that of H. tuberculata, but at the end of gastrulation it begins to generate the precursors of the juvenile adult sea urchin. In contrast the indirect developing pathway at this point begins to generate the feeding pluteus larva. Only much later will adult structures be made. The proposed project has three specific aims: (1) determination of relative roles for cell autonomous versus cell-cell communication in the evolution of early development; (2) identification of regulatory genes that underly the evolutionary divergence of developmental pathways at gastrulation; (3) initiation of studies to experimentally generate evolutionary changes in early development. The study of the evolution of developmental processes offers a means to explore how differences between species arise in the course of evolution. Closely related species that differ in significant ways in developmental pattern provide us with natural varients of developmental processes. The study of these closely related but developmentally distinct species allows mechanisms underlying changes to be identified and their functions determined.